The field of the invention is cardiology. More particularly, the invention relates to a system and method for improving T-wave alternans measurements by excluding certain beats from calculations.
Electrical alternans are the differences in electrical potential at corresponding points between alternate heartbeats. T-wave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or T-wave of an electrocardiogram (ECG) which repeats itself every two beats and has been linked to underlying cardiac instability. Typically, by enumerating all consecutive heart beats of a patient, beats with an odd number are referred to as “odd beats” and beats with an even number are referred to as “even beats.” A patient's odd and even heartbeats may exhibit different electrical properties of diagnostic significance which can be detected by an ECG.
The presence of these electrical alternans is significant because patients at increased risk for ventricular arrhythmia's commonly exhibit alternans in the ST-segment and the T-wave of their ECG. Clinicians may therefore use these electrical alternans as a noninvasive marker of vulnerability to ventricular tacharrhythmias. The term T-wave alternans (TWA) is used broadly to denote electrical alternans such as these. It should be understood that the term encompasses both the alternans of the T-wave segment and the ST-segment of an ECG. U.S. Pat. No. 5,148,812 to Richard L Verrier and Bruce D. Nearing provides an example of a method of quantifying and measuring the magnitude of T-wave alternation in an ECG that can be performed non-invasively.
Determining the magnitude of T-wave alternation can oftentimes be difficult. TWA magnitudes are typically in the range of several microvolts to several hundred microvolts. These small amplitudes make the measurement and analysis of the TWA susceptible to noise. Noise sources such as white noise, motion artifacts caused by respiration or patient movement, noisy heart beats, premature beats and the like can skew TWA measurements.
During a stress test artifacts produced by the pedaling of a bicycle or by the footfalls on a treadmill can significantly disturb the measurement of T-wave alternans. Beats that are highly affected by these artifacts should be excluded from the recorded data, otherwise false positive T-wave alternans may be measured. In addition, when the heart rate approaches the footfall rate or the doubled pedaling rate the superimposed artifacts are difficult to discriminate from a real T-wave alternans. Because the heart rate increases continuously during a stress test, it is likely that the heart rate closely approximates the footfall or doubled pedaling rate for a certain time interval. Accordingly, additional beats must be excluded for a certain period of time after an episode of artifact. Thus, there exists a need to continuously measure the variability (or “variance”) of odd beats and the variability of even beats and exclude specific beats from TWA calculations depending on the measured variability or previous episode of artifacts.